Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is produced by folding and sealing a web of laminated packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, defined for example by a sheet of aluminium, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material eventually forming the inner face of the package contacting the food product.
As is known, such packages are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e.g. vaporized by heating, from the surfaces of the packaging material.
The web of packaging material so sterilized is maintained in a closed sterile environment, and is folded into a cylinder and sealed longitudinally to form a continuous tube. The tube of packaging material is fed in a vertical direction parallel to its axis, is filled continuously with the sterilized or sterile-processed food product, and is fed through a forming station where two or more pairs of forming jaws act cyclically and successively on the tube to seal it along equally spaced cross sections and form a continuous strip of pillow packages connected to one another by respective transverse sealing bands, i.e. extending in a horizontal direction perpendicular to the tube axis.
More specifically, according to a possible known solution, each pair of jaws is hinged at the bottom to a slide running up and down along a vertical guide parallel to the tube of packaging material, so that, as the slide moves vertically between a top dead center position and a bottom dead center position, the jaws open and close substantially “book-fashion” about the tube of packaging material.
To seal the tube of packaging material transversely, the jaws in each pair have respective sealing members cooperating with opposite sides of the tube and defined, for example, by a heating element for locally melting the mutually contacting plastic layers of the packaging material, and by one or more pressure members made of elastomeric material and for providing mechanical support to grip the tube to the required pressure.
More specifically, packaging material in which the barrier layer is defined by a sheet of electrically conducting material, e.g. aluminium, is normally heat sealed by means of a so-called induction heat sealing process, in which, when the two jaws are gripped about the tube, electric current is induced in, and locally heats, the aluminium sheet, thus resulting in localized melting of the heat-seal plastic material.
Upon completion of the heat-seal operation, a cutting member—carried, for example, by one of the two jaws—is activated and interacts with the tube of packaging material to cut it along the centerline of the transverse sealing band and so detach a pillow package from the bottom end of the tube of packaging material. The bottom end is therefore sealed transversely, and, once the slide reaches the bottom dead center position, the jaws open to avoid interfering with the upper part of the tube; and the pillow packages are fed to a final folding station where they are folded mechanically into the finished parallelepiped shape.
In one known solution described and illustrated, for example, in Patent Application EP-A-1172299, the sealing members are generally carried by respective horizontal bar portions of the jaws in each pair, which, above the bar portions, have respective open-fronted, C-section forming tabs which, following heat sealing by the relative sealing members, cooperate with each other to define a cavity of given shape and volume and which encloses and forms the tube of packaging material into a rectangular-section configuration.
More specifically, each forming tab comprises an end wall, from whose opposite lateral edges extend respective parallel lateral walls which cooperate with respective lateral walls of the other forming tab to surround the tube of packaging material.
To control filling of the pillow packages being made, the end walls of each pair of forming tabs have respective convex portions, which exert pressure on or “squeeze” the lateral walls of the packages to limit the amount of food product in and so obtain the desired weight of each package.
Though fairly effective on the whole, the above package filling control system still leaves room for further improvement.
In particular, in the case of small packages, i.e. with small-area lateral walls, to achieve the desired squeezing effect, convex portions must be used which project sharply with respect to the end walls of the relative forming tabs, and which may result in the packages being formed into unacceptable shapes. Moreover, even using sharply projecting convex portions does not ensure the desired weight of the package.